Everyone who applies for a visitor visa to Canada is automatically considered for a multiple entry visa. You don't need to specify it on your application.
The official website of Immigration Canada explains:
You don’t need to choose. A multiple entry visa is what all visa applicants are automatically considered for. We will review your application and issue you a visa depending on your situation.
Multiple entry visa
While valid, a multiple entry visa will let you travel to Canada for six months at a time as many times as you want. It will be valid for up to 10 years or one month before your passport expires, whichever is shorter. You must arrive in Canada on or before the expiry date on your visa.
Single entry visa
A single entry visa lets you travel to Canada only one time. For instance, you may only be eligible for a single entry visa if:
- you are eligible for a fee-exemption and the purpose of your entry to Canada is limited (such as, for an official visit by a foreign national)
- you are taking part in a one-time special event in Canada
- there are approved country-specific procedures or guidelines in place.
In most cases, once you have left Canada, you will need a new visa to enter Canada again.
You won’t need a new visitor visa to return to Canada if you are travelling directly to the United States (including its Territories and Possessions) or St. Pierre and Miquelon.